To prevent a function to be redefined or overwritten, you can write-protect it:
function Test-Function { 'Hello World!' } Set-Item -Path function:Test-function -Options ReadOnly
From this point, your function cannot be overwritten. However, it can be deleted and then redefined. So this is the same level of protection you can expect from the readonly attribute in the file system. It protects you from accidental changes. It won't protect you from intentional changes.
